At some point solid frozen oxygen bars would be useful for humans on planetary missions. Oxygen freezes solid at about 371 degrees below zero f. If it can be stored without loss or sublimation inside some kind of thin wrapper that oxygen might be gathered and frozen on the Moon and Mars. Sending practical oxygen ice making machines to various solar system locations to produce caches of oxygen ice blocks night be an early business opportunity in support of mining and industrial innovation for materials tech.
https://en.wikipedia.org/wiki/Solid_oxygen
http://www.esrf.eu/news/spotlight/spotlight89 the structure of metallic oxygen
https://en.wikipedia.org/wiki/Solid_oxygen
http://www.esrf.eu/news/spotlight/spotlight89 the structure of metallic oxygen
